In recent years, use of a surface acoustic wave (SAW) filter has been spreading in a cellular phone, etc. for the purpose of suppressing undesired signals in transmission and reception, and the demand for the surface acoustic wave filter is increasing. For example, the surface acoustic wave filter is employed in a radio-frequency (RF) circuit section of the cellular phone. Such a surface acoustic wave filter is desired to have characteristics of low insertion loss and wide bandwidth.
FIG. 1 shows a plan view of an exemplary structure of a conventional surface acoustic wave filter. In FIG. 1, the surface acoustic wave filter has a pair of reflective electrodes 2-1, 2-2 on a piezoelectric substrate 1 formed of LiTaO3, LiNbO3, etc.
Also, a plurality of comb electrodes (three electrodes in the example shown in FIG. 1) 3-1, 3-2, 3-3 are structured between the pair of reflective electrodes 2-1, 2-2 on piezoelectric substrate 1. Each comb electrode 3-1, 3-2, 3-3 is constituted of a plurality of finger electrodes.
Among the plurality of finger electrodes constituting comb electrodes 3-1, 3-2, 3-3, finger electrodes are alternately grounded in common, and further, rest of the finger electrodes inserted between the above-mentioned grounded finger electrodes are connected in common to either an input end or an output end.
In the example shown in FIG. 1, alternate finger electrodes provided in the center comb electrode 3-2 are connected in common to an input end IN, while alternate finger electrodes provided in comb electrodes 3-1, 3-3 are connected in common to an output end. Here, each gap between the finger electrodes in comb electrodes 3-1, 3-2, 3-3 (hereafter referred to as electrode pitch) is the same in length.
Through a passband characteristic of the filter, a signal input to the input end IN is characterized, and then output to the output end OUT. At this time, caused by a surface acoustic wave, there are produced a first standing wave A having a resonant wavelength against the entire filter, and a second standing wave B having a resonant wavelength against each comb electrode 3-1, 3-2, 3-3, with an inverse polarity on comb electrode 3-2. Accordingly, in the example shown in FIG. 1, a multimode filter having two resonant modes is structured.
The passband characteristic is determined by the resonant characteristic obtained from the first standing wave A and the resonant characteristic obtained from the second standing wave B.
FIG. 2 shows a surface acoustic wave filter constituted of a plurality of sets of the filter structured of a unit shown in FIG. 1 connected in cascade. By constituting a filter in cascade connection as shown in FIG. 2, it becomes possible to obtain a sharper attenuation characteristic in the frequency band edge portions.
In FIG. 2, although piezoelectric substrate 1 is not shown, two pairs of reflective electrodes (2-1, 2-2), (2-3, 2-4) are structured on one piezoelectric substrate 1, and three comb electrodes (3-1, 3-2, 3-3), (3-4, 3-5, 3-6) are structured between each pair of the reflective electrodes.
This structure shown in FIG. 2 also constitutes a multimode filter having a plurality of resonant modes produced by the standing waves.
FIG. 3 shows an example of passband characteristic in such a multimode filter. In this figure, magnitude of attenuation is on the vertical axis, while frequency is on the horizontal axis. A range between the frequency in the lower zone and the frequency in the higher zone respectively having the magnitude of attenuation of 3 dB below the minimum attenuation value is referred to as a passband width.
As an ideal filter, it is desired to have a filter having a small minimum attenuation value with a wide passband width.
Here, as having been illustrated in FIG. 1, in comb electrodes 3-1, 3-2, 3-3, electrode pitches between the finger electrodes are entirely the same in length, and this is also applicable in comb electrodes 3-4, 3-5, 3-6 in the example shown in FIG. 2.
Further, in order to obtain a wider passband width in the filter characteristic, it has been necessary to set the gaps between the outermost finger electrodes of the respective comb electrodes 3-1 to 3-6, namely the gaps between neighboring comb electrodes, for example, comb electrode 3-1 and comb electrode 3-2, to lengths different from the finger electrode pitch. However, when each gap between the outermost finger electrodes of the comb electrodes is set to a length different from the finger electrode pitch, discontinuity is produced in the phase of a resonant wave at the gap, shown as GAP, between the outermost finger electrodes of the neighboring comb electrodes.
In FIG. 4, there is shown an enlarged diagram of the left portion, halved from the center of the center comb electrode 3-2 in the structure shown in FIG. 1. The right portion thereof is obtained by folding the structure shown in FIG. 4 to the right, which is omitted from the figure.
The lengths of the electrode pitches between the finger electrodes in both comb electrode 3-1 and comb electrode 3-2 are uniform. Further, in FIG. 4, a gap shown as GAP is produced between the outermost finger electrode of comb electrode 3-1 and the outermost finger electrode of comb electrode 3-2. When the gap size, GAP, is not equal to the electrode pitch in comb electrode 3-1 and comb electrode 3-2, discontinuity in phase is produced between the phase of a surface acoustic wave SAW1 on comb electrode 3-1 and the phase of a surface acoustic wave SAW2 on comb electrode 3-2.
This discontinuity in the phases of the surface acoustic waves is illustrated in FIG. 5, which is observed from the cross sectional direction of piezoelectric substrate 1. As shown in FIG. 5, in the gap shown as GAP, discontinuity is produced between the surface acoustic waves SAW1 and SAW2. This brings about producing a bulk wave radiation 4 passing through piezoelectric substrate 1.
This bulk wave radiation 4 becomes a major cause of the insertion loss in the surface acoustic wave filter. Therefore, it is necessary to maintain continuity in the phases of the surface acoustic waves.
As a method for maintaining continuity of the surface acoustic waves, there has been disclosed a technique in German Patent DE 42 12 517. In this technique, electrodes between neighboring comb electrodes are disposed based on a chirp function or a sine wave, and thereby a pseudo repetitive structure is inserted.
However, in general, insertion loss becomes larger as the gap size, GAP between neighboring comb electrodes is set larger. Therefore, it is not preferable that the gap size, GAP, between the neighboring comb electrodes is set large enough to insert such a pseudo repetitive structure as having been disclosed in the above-mentioned German Patent.